The present invention relates to a display device and a method of driving the display device and, in particular, to an active-matrix type organic electroluminescent display device.
An active-matrix type organic electroluminescent display device (hereinafter referred to as AMOLED) is expected as a next-generation flat panel display device.
Among conventional driving circuits for the AMOLED, a two-transistor circuit (hereinafter a first conventional technique) as disclosed in Japanese Patent Application Laid-Open No. 2,000-163,014 (laid open on Jun. 16, 2002) is known as the most fundamental pixel circuit. This two-transistor circuit includes a drive thin film transistor (hereinafter referred to as an EL-drive TFT) for supplying a current to an organic electroluminescent element (hereinafter referred to merely as an EL element), a storage capacitor connected to a gate electrode of the EL-drive TFT for storing a video signal voltage, and a switching thin film transistor (hereinafter referred to as a switching TFT) for supplying a video signal voltage to the storage capacitor.
A major problem that exists in the fundamental two-transistor pixel circuits is nonuniformity in a display that occurs because threshold voltages (Vth) and mobility (μ) of the EL-drive TFTs vary from pixel to pixel due to local variations in the degree of crystallinity of a semiconductor thin film (usually a polysilicon film is used) forming the EL-drive TFTs.
The variations in the threshold voltages and the mobility directly cause variations in the drive currents of the EL elements, and consequently, light emission intensity varies locally, and fine-pattern nonuniformity appears in a display. Such nonuniformity in display becomes pronounced in particular when a halftone display is produced and therefore a drive current is small.
In order to suppress the nonuniformity in display caused by the variations in the characteristics of the EL-drive TFTs, a so-called pulse width modulation driving method (hereinafter a second conventional technique) is disclosed in Japanese Patent Application Laid-Open 2,000-330,527 (laid open on Nov. 30, 2000), for example. In this driving method, EL-drive TFTs are driven as binary switches capable of assuming either of completely OFF and completely ON states, and gray scales in a display is produced by changing durations of light emission.
On the other hand, in general, red-light-emitting, green-light-emitting and blue-light-emitting organic EL elements used for the AMOLED are different from one another in light emission characteristics (light emission luminance, voltage-current characteristics, voltage-light emission luminance (brightness) characteristics, etc.). Also the variations in the light emission characteristics among the red-light-emitting, green-light-emitting and blue-light-emitting organic EL elements are observed as fine-pattern nonuniformity in a display screen as described above. In order to suppress nonuniformity in display due to the variations in the light emission characteristics among the red-light-emitting, green-light-emitting and blue-light-emitting organic EL elements, Japanese Patent Application Laid-Open No. 2,001-92,413 (laid open on Apr. 6, 2001) discloses a method (hereinafter a third conventional technique) which provides a memory storing gamma correction tables for red (R), green (G) and blue (B) video signals to be supplied to red-light-emitting, green-light-emitting and blue-light-emitting organic EL elements, respectively, and selects proper gamma correction values for each of the red (R), green (G) and blue (B) video signals.